Errors

Error Model

Llambda implements the R7RS error system which is built on exceptions raised by (raise) or (error) and caught with (guard). Individual error categories have predicate procedures that can be used to categorise signalled errors inside exception handlers. If an exception is not caught by an exception handler a description of the error will be printed to standard error and the program will terminate.

In this example (open-input-file) is executed and a message is printed whenever a file-error is signalled. If any other error is signalled the exception will be re-raised and handled by the next exception handler.

(guard (condition
         ; Catch file errors and let other errors propagate
         ((file-error? condition)
          (write "Unable to open file!")
          (newline)
          (exit -1)))
       ; Try to open the path under (guard)
       (open-input-file "/some/path"))

If the Llambda compiler determines that code will always cause an error to be signalled it will produce an error message and abort compilation. For example, (/ 1 0) or (abs 'notanumber) would both be rejected as invalid by the compiler. This differs from some Scheme implementations that will only signal errors once the offending code is executed.

Extended Error Categories

R7RS only defines two error categories:

1. file-error is signalled whenever an operation on a file fails. This includes missing files and permission errors.
2. read-error is signalled when the (read) procedure is unable to parse its input.

Llambda extends this error system with a number of additional error categories. Procedures for these extended categories are available in the (llambda error) library. The predicate procedure for an error category is named (category-name?) while the raise procedure is called (raise-category-name). For example, the divide-by-zero-error predicate is (divide-by-zero-error?) and its raise procedure is (raise-divide-by-zero-error).

Error Category	Signalling Condition
arity-error	Procedure called with an incompatible number of arguments
divide-by-zero-error	Integer division by zero was attempted. See the numbers documentation for more information.
implementation-restriction-error	Llambda-specific limitation was encountered in an otherwise valid program
integer-overflow-error	Integer overflow encountered during arithmetic. See the numbers documentation for more information.
invalid-argument-error	Invalid argument supplied to a procedure where a more specific error was not applicable
mutate-literal-error	Attempted mutation of a constant string, pair, vector or bytevector
out-of-memory-error	Process memory exhausted. This is only raised in certain situations; memory exhaustion is usually fatal.
range-error	Collection was indexed outside the bounds of the collection
type-error	Runtime type check failed
undefined-variable-error	Recursive variable referenced before its definition
utf8-error	Invalid UTF-8 encoding was encountered
match-error	Pattern matching failed to match any clauses